'Community' quick action does not display in Cases even when Digital Experiences are enabled

To make the 'Community' quick action available on Cases in Salesforce Classic

1. Navigate to Setup | Customize | Cases | Support Settings.
2. Click Edit.
3. Select Enable Question-to-Case in Salesforce.
4. Click Save.
5. Navigate to the Case object's Page Layouts.
6. Click Edit on the Case layout
7. Click on Quick Actions on the palette.
8. Drag and drop the 'Community' quick action to your Case page layout.
9. Click Save.

To make the 'Community' quick action available on Cases in Lightning Experience

1. Navigate to Setup | Feature Settings | Service | Support Settings.
2. Click on Edit.
3. Select Enable Question-to-Case in Salesforce.
4. Click on Save.
5. Go to Object Manager tab.
6. Click on Case.
7. In the left hand pane, click Case Page Layouts.
8. Click on the Case page layout name.
9. Click on Quick Actions on the palette.
10. Drag and drop the 'Community' quick action to your Case page layout.
11. Click on Save.

Also See:

Enable Case Feed Actions and Feed Items unavailable in Support Settings